Bond Characterization on a Cr–Cr Quintuple Bond: A Combined Experimental and Theoretical Study

Abstract

A combined experimental and theoretical charge density study on a quintuply bonded dichromium complex, Cr(2)(dipp)(2) (dipp = (Ar)NC(H)N(Ar) and Ar = 2,6-i-Pr(2)-C(6)H(3)), is performed. Two dipp ligands are bridged between two Cr ions; each Cr atom is coordinated to two N atoms of the ligands in a linear fashion. The Cr atom is in a low oxidation state, Cr(I), and in low coordination number condition, which stabilizes a metal-metal multiple bond, in this case, a quintuple bond. Indeed, it gives an ultrashort Cr-Cr bond distance of 1.7492(1) Å in the complex. The bond characterization of such a quintuple bond is undertaken both experimentally by high-resolution single-crystal X-ray diffraction and theoretically by density functional calculation (DFT). Electron densities are depicted via deformation density and Laplacian distributions. Bond characterizations of the complex are presented in terms of topological properties, Fermi hole function, source function (SF), and natural bonding orbital (NBO) analysis. The electron density at the Cr-Cr bond critical point (BCP) is 1.70 e/Å(3), quite a high value for metal-metal bonding and mainly contributed from the metal ion itself. The quintuple bond is confirmed with one σ, two π, and two δ interactions by NBO analysis and Fermi hole function. The molecular orbitals (MOs) illustrate that five bonding orbitals are predominantly contributed from the 3d orbitals of the Cr(I) ion. The effective bond order from NBO analysis is 4.60. The detail comparison between experiment and theory will be given. Additionally, three closely related complexes are calculated for systematic comparison.

Strong Chemical Bonds

“…In 2005, Power et al [86] reported the synthesis of the first isolable formal quintuply bonded (σ 2 π 4 δ 4 ) dichromium complex ArCrCrAr (Ar = 2,6-(2,6-iPr 2 C 6 H 3 ) 2 C 6 H 3 ) with an unusually short Cr-Cr distance (1.835 Å). This report stimulated several theoretical and experimental studies [87][88][89][90][91][92][93][94][95] in which the nature of the Cr-Cr bond was probed. Since then, several complexes featuring ultra-short Cr-Cr bonds of around 1.75 Å have been identified [88][89][90][94][95][96].…”

Is it True That the Normal Valence‐Length Correlation Is Irrelevant for Metal–Metal Bonds?

“…[2][3][4] For instance, the length of the CrÀCr quadruple bond for Cr 2 4 + complexesv aries from 1.830 to 2.541 . [5] In addition, for most of the metal-metal bonds, the effectiveb ond order (EBO) is dramatically lower than the FBO. Aw ell-known example is the EBO of 3.2valenceu nits (v.u.)…”

An Electron‐Rich Molybdenum–Molybdenum Quintuple Bond Spanned by One Lithium Atom

“…The existence of the CrÀCr quintuple bond of the type II complex was recently corroborated by charge-density studies. [9] Preliminary reactivity studies on the type II complexes indicate these univalent and low-coordinate dinuclear species are reactive towards unsaturated organic functionalities and small molecules. [10,11] Compared to the many type II complexes, only one example of the type III complex, [Cr 2 {m-HC(N-2,6-Me 2 C 6 H 3 ) 2 } 3 ] À (1), was recently reported, although such type of species was proposed by Hoffmann et al in 1979.…”